Cyclopentane

Cyclopentane (also called C pentane) is a highly flammable alicyclic hydrocarbon with chemical formula C5H10 and CAS number 287-92-3, consisting of a ring of five carbon atoms each bonded with two hydrogen atoms above and below the plane. It occurs as a colorless liquid with a petrol-like odor. Its freezing point is −94 °C and its boiling point is 49 °C. Cyclopentane is in the class of cycloalkanes, being alkanes that have one or more carbon rings. It is formed by cracking cyclohexane in the presence of alumina at a high temperature and pressure.

It was first prepared in 1893 by the German chemist Johannes Wislicenus.

Production, occurrence and use
Cycloalkanes are formed by catalytic reforming. For example, when passed over a hot platinum surface, 2-methylbutane converts into cyclopentane.

Cyclopentane is principally used as a blowing agent in the manufacture of polyurethane insulating foam, replacing ozone-depleting agents such as CFC-11 and HCFC-141b. While cyclopentane is not typically used as a refrigerant, it is common for domestic appliances that are insulated with cyclopentane-based foam, such as refrigerators and freezers, to be marked with cyclopentane warning labels due to its flammability. Cyclopentane is also used in the manufacture of synthetic resins and rubber adhesives.

Multiply alkylated cyclopentane (MAC) lubricants, such as 1,3,4-tri-(2-octyldodecyl) cyclopentane, have low volatility and are used by NASA in space applications.

Cyclopentane requires safety precautions to prevent leakage and ignition as it is both highly flammable and can also cause respiratory arrest when inhaled.

Cyclopentane can be fluorinated to give compounds ranging from C5H9F to perfluorocyclopentane C5F10. Such species are conceivable refrigerants and specialty solvents.

The cyclopentane ring is pervasive in natural products including many useful drugs. Examples include most steroids, prostaglandins, and some lipids.

Conformations
In a regular pentagon, the angles at the vertices are all 108°, slightly less than the bond angle in perfectly tetrahedrally bonded carbon, which is about 109.47°. However, cyclopentane is not planar in its normal conformations. It puckers in order to increase the distances between the hydrogen atoms (something which does not happen in the planar cyclopentadienyl anion C5H5- because it doesn't have as many hydrogen atoms). This means that the average C-C-C angle is less than 108°. There are two conformations that give local minima of the energy, the "envelope" and the "half-chair". The envelope has mirror symmetry (C$s$), while the half chair has two-fold rotational symmetry (C$2$). In both cases the symmetry implies that there are two pairs of equal C-C-C angles and one C-C-C angle that has no pair. In fact for cyclopentane, unlike for cyclohexane (C6H12, see cyclohexane conformation) and higher cycloalkanes, it is not possible geometrically for all the angles and bond lengths to be equal except if it is in the form of a flat regular pentagon.